1. Field of the Invention
The present invention relates to lithographic illumination systems and methods and, more particularly, to multiple radiation source beams projected onto a common beam path.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., a resist). Instead of a mask, the patterning means may comprise a contrast device, such as an array of individually controllable elements that serve to generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions to be exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated an entire pattern onto the target portion in a single exposure, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Lithographic apparatus are known in which a pattern is imparted to a projection beam by an array of individually controllable elements. Thus, in such apparatus, rather than relying upon a preformed mask (also referred to as a reticle) to impart a pattern to a beam, control signals are delivered to the array of controllable elements so as to control the state of those elements thereby to pattern a projection beam. Such apparatus is generally referred to as “maskless” given that it relies upon individually controllable elements rather than a mask to impart the necessary pattern to the projection beam.
Maskless lithographic apparatus can be used to expose relatively large area substrates, for example substrate to be used as flat panel displays. Such panels may be exposed in a single pass beneath an array of projection systems, each of which is provided with its own patterning system incorporating an array of individually controllable elements. As the substrate is displaced relative to the projection systems, it is necessary to change the state of individual elements in the arrays of controllable elements so as to change the projected patterns. The rate at which the state of the individual elements can be changed (generally referred to as the update rate) is limited, and this imposes an upper limit on the maximum speed at which a substrate can be displaced relative to the projection systems. The speed of displacement determines the maximum throughput of the apparatus.
The speed of displacement is also dependent upon the intensity of the projection beam used to irradiate the substrate. For a given intensity projection beam, the larger the area of substrate exposed by the beam, then the smaller the power/intensity density of the radiation beam per unit area of substrate.
In order to transfer the desired pattern to the substrate, it is necessary to provide a predetermined total amount of radiation energy per unit area of substrate formed. This predetermined total amount will vary, depending upon the wavelength or type of radiation beam, and the type and thickness of the resist material. If the power per unit area of substrate if decreased, then a longer exposure time is required. Thus, the projection beam power also determines the maximum throughput of the apparatus.
However, it is not always possible to provide a single radiation source having the desired power output. The power output of radiation sources can be limited by technology and/or the cost of high power radiation sources can be relatively expensive.
There is a need, therefore, for improved lithography illumination systems and methods.